The plating of plastic parts is well known and of considerable commercial importance because the desirable characteristics of both the plastic and the metal are efficiently utilized in many applications. Thus a significant demand exists for metal plated plastic parts of all types. This includes not only plated plastic parts in general, but also the plated through-holes of printed circuit boards. In general, the process of plating plastics consists of etching the plastic surface followed by conditioning, activation and metallization.
The problems in plating plastics are well known in the art. One of the most serious problems is providing a plate which is sufficiently adherent to the plastic surface. Another equally troublesome problem is providing for complete and even coverage of the plastic surface. Various methods have been employed in the past to rectify these problems. One approach is to provide for a plastic surface pre-etch sensitizer which will sensitize the plastic surface and thereby allow the subsequent plastic etch to more effectively etch the plastic surface. It has been demonstrated that through this approach of etching a sensitized plastic surface more effectively both improved coverage and adhesion can be achieved. It is thought that improved coverage is achieved in this way because the rougher plastic surface adsorbs more plating catalyst and thereby plates more efficiently. It is also thought that the increased adhesion of the plate to the surface is due to the increased roughness of the etched plastic surface.
Various sensitizers are known in the art. U.S. Pat. No. 3,758,332 discloses the use of chemicals such as methyl ethyl ketone, tetrahydrofuran, dioxane, pyridine, dimethylformamide and various alcohol mixtures. U.S. Pat. No. 4,125,649 reveals the use of halogenated alcohols, ketones and aldehydes as sensitizers for polysulfone resins. U.S. Pat. No. 4,775,557 discloses the use of a compound of the general formula, R.sub.1 (OC.sub.n H.sub.2n).sub.m O-R.sub.2 as a sensitizer for polycarbonate resins, wherein R.sub.1 and R.sub.2 are selected from aryl or alkyl groups containing from 1-4 carbon atoms, n is 2 to 4 and m is 1 to 5. Finally U.S. Pat. Nos. 3,579,365 and 3,563,784 disclose an aqueous emulsion of turpentine with various surfactants as a sensitizing solution for acrylonitrile-butadiene-styrene (ABS). Various etchants for plastic which follow the sensitizer are known in the prior art including sulfuric acid, chromic acid, and permanganate solutions. These and other etch solutions are used following a sensitizer to etch and roughen the surfaces of various plastics in order to prepare these plastic surfaces for plating.
These sensitizers of the prior art have achieved mixed results. They have generally been moderately effective at increasing the coverage and adhesion of plated metal deposits on plastic substrates. Many of the prior art sensitizers were directed to ABS type plastics and are completely ineffective in sensitizing the polycarbonates and polycarbonate alloys which are extensively used today. But the most significant drawback of these prior art sensitizers is that they are relatively dangerous to handle and environmentally unsafe.